Passenger vehicles and trucks incorporate a powertrain having a multi-speed transmission. Initially, these transmissions were countershaft-type transmissions with very few planetary gearsets. As time progressed, however, planetary gearsets became the more popular powertrain option, particularly in the United States.
These transmissions, both planetary and countershaft, have been developed more recently to provide more drive ratios that are selectively operable between an engine and a powertrain output. The number of ratios in these transmissions has increased from two to seven or more ratios. With each additional ratio, an additional ratio interchange is required, which requires interchanges of torque-transmitting mechanisms (clutches and brakes) and the resulting speed changes of transmission elements as well as engine speed changes.
To reduce the number of ratio interchanges or interchange events, it has been considered that variable speed transmissions would be advantageous. Variable speed transmissions have continuous ratio change such that a ratio interchange event is not evident. Most variable transmissions or continuously-variable transmissions (CVTs) have taken the form of hydraulically-variable units, electrically-variable units, and friction belt units.
Friction belt units have been used in many small implement applications such as tractors and snow machines and currently become popular with automobiles. The variable speed transmissions lend themselves well to improved vehicle economy and vehicle performance. In the line of vehicle economy, the engine can be operated at the most efficient fuel consumption rate while the vehicle ratio in the powertrain is adjusted by using the transmission. With a continuously variable transmission, the requirement of having a fixed ratio for an area of performance is no longer required.